In a magnetic resonance imaging apparatus, especially in an RF coil, a balun (balance/unbalance) circuit is used as one means for reducing the noise and/or stray RF currents generated due to induced currents in a co-axial cable.
Known configurations of a balun circuit, especially for head coil in a magnetic resonance apparatus, include a box type-balun and a stripline balun for providing isolation between the grounds of the balanced and unbalanced end of the co-axial cable.
However, for high field magnetic resonance apparatus e.g. operating at a magnetic field strength of about 3 tesla, the ground shield currents are very high, and these known configurations (i) do not provide sufficient ground-to-ground isolation and also (ii) do not withstand temperature rise predetermined to meet the heat test specifications and optimum functioning of the balun circuit.
Thus, these known configurations do not effectively eliminate shield current (stray RF current) and hence does not contribute to sufficient noise reduction in co-axial cable transmission line.